Non-sag incandescent tungsten filament for an incandescent lamp

ABSTRACT

Incandescent lamp incorporates a non-sag tungsten filament having a microstructure comprising a plurality of elongated and interlocking grains disposed along the axial dimension of the filament. A large number of very minute inert gas bubbles, such as helium bubbles, are included within the filament, and a portion of the bubbles are generally aligned along the axial dimension of the filament and are positioned at the boundaries of the interlocking grains which comprise the filament. In the method for making the filament, inert gas atoms, such as helium atoms, are formed within the tungsten and when the filament is incandesced, the atoms migrate to form the very minute helium bubbles.

FILAMENT FOR AN INCANDESCENT LAMP Inventors: Heinz G. Sell, Cedar Grove,N..I.;

Heinz-J. Stepper, Konigsbrunn, Germany Assignee: Westinghouse ElectricCorporation,

Pittsburgh, Pa.

Filed: Jan. 12,1972 Appl. No.: 217,162

US. Cl 313/217, 313/218, 313/222, 313/315, 313/344 Int. Cl. H0lk 1/04Field of Search... 313/222, 315, 217, 218, 311, 313/344, 343

United States Patent 91 [111 3,789,255 Sell et al. i I r Jan. 29, 1974NON-SAG INCANDESCENT TUNGSTEN Primary Examiner-Palmer C. Demeo Attorney,Agent, or Firm-A, T. Stratton et a1.

Incandescent lamp incorporates a non-sag tungsten filament having amicrostructure comprising a plurality of elongated and interlockinggrains disposed along the axial dimension of the filament. A largenumber of very minute inert gas bubbles, such as helium bubbles, areincluded within the filament, and a portion of the bubbles are generallyaligned along the axial dimension of the filament and are positioned atthe boundaries of the interlocking grains which comprise the filament.In the method for making the filament, inert gas atoms, such as heliumatoms, are formed within the tungsten and when the filament isincandesced, the atoms migrate to form the ery minute helium bubbles.

ABSTRACT 6 Claims, 4 Drawing Figures PAIENTEB JAN 291974 SHEET 2 OF 2FIG. 4

BACKGROUND OF THE INVENTION This invention generally relates toincandescent lamps and, more particularly, to an incandescent lamp whichincorporates an improved non-sag tungsten filament. There is alsoprovided a method for making the filament.

Non-sag tungsten filaments were initially developed prior to 1920 andthe introduction of the non-sag tungsten filament, as described in US.Pat. No. 1,410,499 dated Mar. 21, 1922, had a great impact on theincandescent lamp industry. In explanation of the term nonsag, if atungsten filament elongates when. it is operated, and thus sags, theindividual turns between filament coils will tend to contact one anotherand short out, thereby shortening the life of the filament. In thepractices of the prior art, non-sag tungsten has been fabricated byincluding therein a very small amount of so-called dopant, which dopantis added in the form In processing tungsten, the material is initiallyformed into a relatively massive ingot by conventional self-resistancesintering techniques. Thereafter, the relatively massive ingot ismechanically reduced in cross section, first by swaging and then bydrawing into filamentary form. In accordance with the method for makingthe present filament, after the relatively massive sintered ingot hasbeen formed, and before the resulting elongated tungsten filamentarymember has been heated to a condition of incandescence, there is formedwithin the tungsten body a very large number of inert gas atoms.Thereafter, after the elongated filamentary member has been fabricatedinto the form intended for of alkali silicates. Because of this dopantaddition,

when the filament is initially incandesced, the crystals or grains whichare formed therein are elongated and interlocking and generally disposedalong the axialdimension of the filament. Within recent years, it hasbeen discovered that the non-sag characteristic which is imparted by thealkali silicate doping is due to the formation of veryminute potassiumparticles, which form potassium bubbles when the filament isincandesced. A substantial portion of these bubbles are generallyaligned along the axial dimension of the filament and serve to inhibitthe recrystallization and control the grain growth of the filament dueto dislocation and grain boundary pinning by the potassium bubbles.

These potassium-formed bubblesare generally submicroscopic in nature anda representative diameter of same is in the order of 100 to 500A. Whilethe performance of such a-filament is generally satisfactory, a majorproblem in the manufacture of the tungsten is the uniform incorporationof the doping impurity into the metal, and the attainment of theconsistency of the required concentration, in orderto produce the bestfilament material.

'- It is disclosed by C. E. Ellis in ACTA Metallurgica, Volume 11,February 1963 that helium filled bubbles formed in samples ofcold-worked aluminum by cyclotron alpha irradiation have an effect onsubsequent recrystallation and grain growth in the aluminum samples. Thehelium bubbles also inhibit grain growth in beryllium andrecrystallization in zirconium.

SUMMARY OF THE INVENTION In accordance with the present invention, thereis provided, in combination with an incandescent lamp, an elongatedtungsten filament having a microstructure which comprises a plurality ofelongated and interlocking grains defined by boundries therebetween. Theelongated dimension of the grains is disposed along the axial dimensionof the filament. A very large number of very minute,'individual inertgas bubbles are included within the filament, and a portion of theindividual bubbles are generally aligned along the axial dimension ofthe filament and positioned at boundries of the interlocking grains. Theresulting structure operates with a non-sag characteristic.

its ultimate use, such as a coil or a coiled-coil, and also mounted inthe environment in which it is intended to operate, the filamentarymember is heated to a condition of incandescence which causes the inertgas atoms to coalesce into a very large number of very minute inert gasbubbles, which causes the filament to recrystallize with an interlockingand non-sag grain structure. In one method for forming the inert gasatoms coalesce to form helium bubbles.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of theinvention, reference should be had to the preferred embodiment,exemplary of the-invention, shown in the accompanying drawings in which!FIG. 1 is an elevational view, partly in section, illustrating anincandescent lamp which incorporates the non-sag tungsten filament whichhas been processed in accordance with the present invention; 9

FIG. 2 is a greatly enlarged sketch showing a crosssection of arecrystallized tungsten filament which includes helium bubbles in thebody thereof, with a portion of the bubbles positioned at the boundriesof the overlappinggrains which comprise the filament;

FIG. 3 is a transmission electron micrograph of alpha-particleirradiated pure tungsten annealed at l,900C for 45 minutes showinghelium bubbles having a diameter of approximately 200A.; and

FIG. 4 is a photomicrograph (taken at 1,000 X) of alpha-particleirradiated pure tungsten ribbon annealed at 2,200C for 45 minutesshowing the elongated, interlocking grain structure developed in theirradiated layer.

DESCRIPTION OF THE PREFERRED EMBODIMENT As shown in FIG. 1, thegenerally conventional incandescent lamp 10 comprises a sealedradiationtransmitting envelope 12 which encloses a predeterminedenvironment, such as an argon atmosphere, in which an incandescentfilament 14 can be operated. Lead-in conductors 16 are sealed throughand extend into the envelope 12 and these electrically connect to aconventional standard screw base 18. The envelope 12 can carry thereonan internal coating 20 of lightscattering material, if desired. Thefilament 14 has been fabricated in accordance with the teachings of thepresent invention, as explained in detail hereinafter. The filament 14can be formed as a single coil of tungsten wire or as a coil which inturn is formd into a coil to form a so-called coiled-coil. Of course,other filament constructions are possible, as are well known in the artand the gaseous atmosphere can be replaced by a vacuum or by acombination of inert gas and halogen, and such operating atmospheres arewell known.

In one method of making the non-sag filament of the present invention, avery small amount of boron (isotope B is mixed with the-tungsten powderprior to forming a compact of tungsten. As an example, the boron isotopecan be included in an amount of about 0.5 percent by weight. The amountof added boron isotope does not appear to be critical. The formedtungsten compact is then pre-sintered by heating to a temperature suchas 1,000C in a non-reactive atmosphere for about one-half hour. Thepre-sintered compact is then self-resistance sintered in accordance withconventional techniques, in order to form a relatively massive sinteredtungsten ingot. The sintering atmosphere is preferably hydrogen. Theformed tungsten ingot is then swaged or otherwise mechanically workeduntil it is greatly elongated, with a relatively small diameter such as0.083 inch (2.1 mm). The elongated material is then drawn to wire ofdesired size.

In accordance with the present invention, after the tungsten ingot hasbeen mechanically reduced to a relatively small diameter, such as wirehaving a diameter of 0.1 inch (2.54 mm) it is passed through anirradiating chamber and irradiated with neutrons. The following nuclearreactions which occur can be described as follows:

W B l, L3 Li "LP," 2He After the foregoing irradiation, the tungsten isworked and drawn to wire of the desired size, and an example is 7 mils(0.178 mm) tungsten wire. The wire is formed into a coil or acoiled-coil as desired. This tungsten wire will exhibit a microstructurewhich comprises what is known in the art as a worked structure due tothe swaging and the drawing operations, and the wire is ductile in thatit can be wound into coils, with the formed helium atoms as well aslithium atoms distributed throughout the tungsten wire. When thetungsten filament is initially incandesced under-predeterminedcontrolled conditions, it will begin to recrystallize and the heliumatoms, as well as residual lithium atoms, which are distributedthroughout the tungsten will begin to coalesce to form small bubbles ofhelium and lithium. These bubbles will be discrete and will be generallyaligned in the direction of the axis of the wire.

As such, they will function similarly to the potassium bubbles of thestandard dopedtungsten and will retard recrystallization and controlsame due to the effect of dislocation and grain boundary pinning.

An enlarged sketch of the present wire is disclosed in FIG. 2 whereinthe inert gas bubbles such as helium bubbles 22 are dispersed throughoutthe body of the tungsten filament 14, with a portion of the bubblesgenerally aligned along the axial dimension of the filament andpositioned at the boundries 24 of the overlapping elongated andinterlocking grains 26 which comprise the filament. 1f alkalimetal-formed bubbles 28 are also present, they will have the generalappearance and orientation as the helium bubbles.

As a second method for forming the inert gas bubbles, after the filamenthas been drawn to a relatively fine size, such as a O. linch (2.54 mm),for example, it is irradiated with alpha particles which forms heliumatoms within the body of the tungsten. Thereafter, the filament is drawnto its desired ,size and then recrystallized to an interlockingstructure.

In FIG. 3 is shown a transmission electron micrograph of alpha-particleirradiated pure tungsten which has later been annealed at a temperatureof 1 ,900C for approximately 45 minutes, with the magnification in thiselectron micrograph being 60,000. Each of the plurality of heliumbubbles which are shown have a diameter of approximately 200A. Thehelium atoms were introduced into the tungsten by irradiating tunstenribbon with 30 MeV alpha particles. The helium bubbles began to nucleateor coalesce at an annealing temperature of approximately 1,600C and thebubbles continued to grow in size as the annealing temperature wasincreased. After an anneal at approximately 2,200C, the average size ofthe bubbles increased somewhat with the majority of the bubbles having adiameter substantially less than one micron.

To show the difference between tungsten which had been irradiated withthe alpha particles to form the helium atoms therein, as compared tosubstantially pure tungsten which contained no helium, a narrow zone ofa tungsten ribbon was irradiated with a 30 MeV alpha particles, with theresulting structure shown in FIG. 4, which is a photomicrograph taken ata magnification of 1,000 X. The irradiated zone occurs in this center ofthis photomicrograph and comprises an elongated, interlocking crystalstructure comprising a plurality of elongated and interlocking grainsdefined by boundries therebetween. A very large number of very minuteindividual helium bubbles are included within the tungsten, and aportion of the bubbles are generally aligned along the grain boundries.The portion of the tungston which was not irradiated with the alphaparticles began to recrystallize at annealing temperatures in the orderof 1,300C to 1,4()0C, but the recrystallization process was retarded inthe irradiated band and initiation of recrystallization was not observeduntil an annealing temperature 2,200C was reached.

The concentration of the helium bubbles within the tungsten body doesnot appear to be particularly critical and can vary over a wide range.As an example, for helium bubbles having a diameter generally in theorder of 2,600A., a bubble concentration per cubic centimeter oftungsten in the order of from about 4 X 10 to 5 X 10 provides very goodresults.

Inert gases other than helium can be formed within the tungsten body. Asan example, if *Mg is included within the tungsten body and irradiatedwith neutrons, it will form within the tungsten a mixture of neon,helium and the alkali metal sodium. The latter will form sodium asbubbles in the tungsten body when the body is incandesced, in a mannersimilar to potassium. The Mg can be used to replace the boron isotope orit can be used to supplement same to form helium and a mixture oflithium and sodium. The alkali metal-formed bubble will be present invery large numbers and will be of a very minute size, such as in theorder of to 500 A. As another example, Al irradiated with neutrons willform Na" and helium atoms. Detailed explanations of such nucleartransmutations are described in Nuclear Physics (2nd Ed), by IrvingKaplan, Addison- Wesley Publishing Co., Inc. (1964).

We claim: I

1. In combination with an incandescent lamp comprising a sealedradiation-transmitting envelope enclosing a predetermined environment inwhich an incandescent filament can be operated, and lead-in conductorssealed through and extending into said envelope, the improvedincandescent lamp member which comprises:

a. an elongated tungsten filament supported within said envelope andelectrically connected to said lead-in conductors, and

b. the microstructure of said filament comprising a plurality ofelongated and interlocking grains defined by boundries therebetweeen andhaving their elongated dimension along the axial dimension of saidfilament, a very large number of very minute individual inert gasbubbles included within said filament, and a portion of said individualbubbles generally aligned along the axial dimension of said filament andpositioned at the boundries of said grains which comprises saidfilament.

2. The incandescent lamp combination as specified in claim 1, whereinsaid inert gas is helium.

3. The incandescent lamp as specified in claim 2, wherein the majorportion of said inert gas bubbles have a diameter substantially lessthan one miron.

4. The incandescent lamp combination as specified in claim 3, whereinthe bubble concentration per cubic centimeter is in the order of about 4X l0 to 5 X l0 bubbles.

5. The incandescent lamp combination as specified in claim 1, wherein avery large number of very minute alkali metal-formed bubbles are presentand are disposed in a manner similar to said inert gas bubbles.

6. The incandescent lamp combination as specified in claim 5, whereinsaid alkali metal-formed bubbles are at least one of lithium-formedbubbles and sodiumformed bubbles.

1. In combination with an incandescent lamp comprising a sealedradiation-transmitting envelope enclosing a predetermined environment inwhich an incandescent filament can be operated, and lead-in conductorssealed through and extending into said envelope, the improvedincandescent lamp member which comprises: a. an elongated tungstenfilament supported within said envelope and electrically connected tosaid lead-in conductors, and b. the microstructure of said filamentcomprising a plurality of elongated and interlocking grains defined byboundries therebetweeen and having their elongated dimension along theaxial dimension of said filament, a very large number of very minuteindividual inert gas bubbles included within said filament, and aportion of said individual bubbles generally aligned along the axialdimension of said filament and positioned at the boundries of saidgrains which comprises said filament.
 2. The incandescent lampcombination as specified in claim 1, wherein said inert gas is helium.3. The incandescent lamp as specified in claim 2, wherein the majorportion of said inert gas bubbles have a diameter substantially lessthan one miron.
 4. The incandescent lamp combination as specified inclaim 3, wherein the bubble concentration per cubic centimeter is in theorder of about 4 X 1011 to 5 X 1011 bubbles.
 5. The incandescent lampcombination as specified in claim 1, wherein a very large number of veryminute alkali metal-formed bubbles are present and are disposed in amanner similar to said inert gas bubbles.
 6. The incandescent lampcombination as specified in claim 5, wherein said alkali metal-formedbubbles are at least one of lithium-formed bubbles and sodium-formedbubbles.